This invention relates generally to aircraft gas turbine engines, and more specifically to counter-rotating gas turbine engines.
At least some known gas turbine engines include a forward fan, a core engine, and a power turbine. The core engine includes at least one compressor, a combustor, a high-pressure turbine, and a low-pressure turbine coupled together in a serial flow relationship. More specifically, the compressor and high-pressure turbine are coupled through a high-pressure shaft to define a high-pressure rotor. The compressor compresses air entering the core engine that is then mixed with fuel and ignited to form a high energy gas stream. The gas stream flows through the high-pressure turbine, rotatably driving it and the high-pressure shaft that, in turn, rotatably drives the compressor.
The gas stream is expanded as it flows through the low-pressure turbine. The low-pressure turbine rotatably drives the fan through a low-pressure shaft such that a low-pressure rotor is defined by the fan, the low-pressure shaft, and the low-pressure turbine. At least some known low pressure turbines include counter-rotating turbines that power counter-rotating fans and counter-rotating boosters and/or low pressure compressors.
When operating such counter-rotating turbines, torque is split substantially equally between the forward and aft fan shafts to facilitate optimizing the efficiency of such turbines. Moreover, engine performance may be improved, for example, by operating the forward fan at a higher fan pressure ratio and/or higher rotational speed than the aft fan. However, operating the aft fan at a lower fan speed and/or a lower fan pressure ratio may cause booster stages that are driven off the aft fan to operate below a peak efficiency and/or reduce the performance of the low-pressure turbine driving the aft fan.